doc: update release tag in source directions to go1.6
doc: update release tag in source directions to go1.6

Fixes #14371

Change-Id: I8e9e4b2b89083f5a947e791c011912fdf365a11e
Reviewed-on: https://go-review.googlesource.com/19996
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

Cofyc/go,cloudflare/go,akutz/go,codestation/go,cloudflare/go,mwhudson/go,ganboing/go-esx,vsekhar/elastic-go,akutz/go,4ad/go,vanloswang/go,4ad/go,Samurais/go,codestation/go,codestation/go,goodlang/good,momchil-velikov/go,cloudflare/go,Samurais/go,golang/go,ImJasonH/go,miolini/go,linux-on-ibm-z/go,cloudflare/go,cloudflare/go,momchil-velikov/go,momchil-velikov/go,golang/go,Cofyc/go,ImJasonH/go,suncycheng/go,polyverse-security/go,vanloswang/go,polyverse-security/go,goodlang/good,ImJasonH/go,CAFxX/go,josharian/go,akutz/go,codestation/go,theif519/go,codestation/go,4ad/go,Samurais/go,vanloswang/go,Samurais/go,suncycheng/go,akutz/go,vanloswang/go,Cofyc/go,josharian/go,polyverse-security/go,mwhudson/go,goodlang/good,binarycrusader/go,cloudflare/go,miolini/go,christopher-henderson/Go,ganboing/go-esx,Samurais/go,goodlang/good,Cofyc/go,derekmarcotte/go,linux-on-ibm-z/go,ganboing/go-esx,nawawi/go,nawawi/go,binarycrusader/go,akutz/go,derekmarcotte/go,goodlang/good,golang/go,Cofyc/go,cloudflare/go,polyverse-security/go,ganboing/go-esx,CAFxX/go,vanloswang/go,4ad/go,vanloswang/go,4ad/go,binarycrusader/go,theif519/go,Samurais/go,nawawi/go,josharian/go,suncycheng/go,momchil-velikov/go,ImJasonH/go,nawawi/go,theif519/go,goodlang/good,josharian/go,derekmarcotte/go,codestation/go,derekmarcotte/go,mwhudson/go,binarycrusader/go,goodlang/good,CAFxX/go,polyverse-security/go,nawawi/go,derekmarcotte/go,polyverse-security/go,derekmarcotte/go,mwhudson/go,Cofyc/go,binarycrusader/go,ganboing/go-esx,theif519/go,suncycheng/go,Samurais/go,theif519/go,vanloswang/go,miolini/go,CAFxX/go,derekmarcotte/go,akutz/go,cloudflare/go,ImJasonH/go,mwhudson/go,theif519/go,suncycheng/go,akutz/go,momchil-velikov/go,vsekhar/elastic-go,linux-on-ibm-z/go,vsekhar/elastic-go,codestation/go,codestation/go,mwhudson/go,vanloswang/go,momchil-velikov/go,miolini/go,nawawi/go,golang/go,goodlang/good,josharian/go,miolini/go,linux-on-ibm-z/go,vsekhar/elastic-go,ImJasonH/go,ImJasonH/go,4ad/go,suncycheng/go,golang/go,binarycrusader/go,vsekhar/elastic-go,CAFxX/go,4ad/go,binarycrusader/go,ganboing/go-esx,vsekhar/elastic-go,miolini/go,miolini/go,miolini/go,vsekhar/elastic-go,binarycrusader/go,polyverse-security/go,nawawi/go,golang/go,josharian/go,suncycheng/go,josharian/go,ganboing/go-esx,CAFxX/go,golang/go,momchil-velikov/go,momchil-velikov/go,linux-on-ibm-z/go,polyverse-security/go,CAFxX/go,ganboing/go-esx,theif519/go,theif519/go,golang/go,ImJasonH/go,vsekhar/elastic-go,suncycheng/go,linux-on-ibm-z/go,mwhudson/go,nawawi/go,derekmarcotte/go,4ad/go,linux-on-ibm-z/go,Cofyc/go,akutz/go,linux-on-ibm-z/go,josharian/go,Cofyc/go,CAFxX/go,mwhudson/go,Samurais/go